The increasing noise susceptibility of many digital circuits and the requirement to comply with international standards mean that electromagnetic compatibility is an increasingly important issue. One way of improving electromagnetic compatibility is to reduce the level of electromagnetic interference (EMI) noise present on the power lines by filtering the noise.
In a power supply with two or more power lines, the EMI noise can include common mode (CM) noise and differential mode (DM) noise. The CM noise and DM noise are defined relative to protective earth, although a protective earth connection may not be present in the system. CM noise is a potential or current that is present in both lines, with no phase shift between the two lines. DM noise is a potential or current that is present in both lines, with a phase difference of 180° between the two lines.
It is known to filter CM noise and DM noise with passive filters. These are cheap and robust, but take up a lot of space on a printed circuit board compared to other components. Passive filters also reduce the efficiency of the whole system due to their energy consumption.
Active filters are envisaged that take up less space and might be more efficient. For an active filter to work, noise detection circuits (also referred to as sensors) measure the level of CM noise and DM noise, and then process the signal to cancel out or mitigate the detected noise signals. CM noise and DM noise require different cancellation and mitigation techniques, and so it is necessary to measure the separate CM noise signal and DM noise signal.
Existing CM noise detection circuits or sensors either make use of a connection to protective earth or use a transformer. Not all devices in which a CM noise detection circuits or sensors may be implemented have a protective earth connection available and transformer based CM noise sensors might be bulky and/or expensive.
At least one object of the invention is to provide a CM noise detection circuit for use with an active CM noise filter.